Effect Of Surface Charge Distribution Research Articles

Adsorption of a Polyelectrolyte Chain at Dielectric Surfaces: Effects of Surface Charge Distribution and Relative Dielectric Permittivity

Adsorption of a Polyelectrolyte Chain at Dielectric Surfaces: Effects of Surface Charge Distribution and Relative Dielectric Permittivity

Effect of the surface charge distribution on the fluid phase behavior of charged colloids and proteins.

A generic but simple model is presented to evaluate the effect of the heterogeneous surface charge distribution of proteins and zwitterionic nanoparticles on their thermodynamic phase behavior. By considering surface charges as continuous "patches," the rich set of surface patterns that is embedded in proteins and charged patchy particles can readily be described. This model is used to study the fluid phase separation of charged particles where the screening length is of the same order of magnitude as the particle size. In particular, two types of charged particles are studied: dipolar fluids and protein-like fluids. The former represents the simplest case of zwitterionic particles, whose charge distribution can be described by their dipole moment. The latter system corresponds to molecules/particles with complex surface charge arrangements such as those found in biomolecules. The results for both systems suggest a relation between the critical region, the strength of the interparticle interactions, and the arrangement of charged patches, where the critical temperature is strongly correlated to the magnitude of the dipole moment. Additionally, competition between attractive and repulsive charge-charge interactions seems to be related to the formation of fluctuating clusters in the dilute phase of dipolar fluids, as well as to the broadening of the binodal curve in protein-like fluids. Finally, a variety of self-assembled architectures are detected for dipolar fluids upon small changes to the charge distribution, providing the groundwork for studying the self-assembly of charged patchy particles.

Effects of Acids, Bases, and Heteroatoms on Proximal Radial Distribution Functions for Proteins.

The proximal distribution of water around proteins is a convenient method of quantifying solvation. We consider the effect of charged and sulfur-containing amino acid side-chain atoms on the proximal radial distribution function (pRDF) of water molecules around proteins using side-chain analogs. The pRDF represents the relative probability of finding any solvent molecule at a distance from the closest or surface perpendicular protein atom. We consider the near-neighbor distribution. Previously, pRDFs were shown to be universal descriptors of the water molecules around C, N, and O atom types across hundreds of globular proteins. Using averaged pRDFs, a solvent density around any globular protein can be reconstructed with controllable relative error. Solvent reconstruction using the additional information from charged amino acid side-chain atom types from both small models and protein averages reveals the effects of surface charge distribution on solvent density and improves the reconstruction errors relative to simulation. Solvent density reconstructions from the small-molecule models are as effective and less computationally demanding than reconstructions from full macromolecular models in reproducing preferred hydration sites and solvent density fluctuations.

Self-Assembly of Patterned Nanoparticles on Cellular Membranes: Effect of Charge Distribution

Nanoparticle-assisted drug delivery has been emerging as an active research area. Achieving high drug loading is only one facet of drug delivery issues; it is also important to investigate the effect of surface charge distribution on self-assembly of nanoparticles on cellular membranes. By considering the electrostatic distribution of patterned nanoparticles, we used dissipative particle dynamics simulations to investigate the self-assembly of pattern charged nanoparticles with five different surface charged patterns. It is found that both surface charged pattern and nanoparticle size significantly affect the self-assembly of nanoparticles on cellular membranes. Results indicate that 1/2 pattern charged small nanoparticles can self-assemble into dendritic structures, while those with a 1/4 pattern self-assemble into clusters. As the nanoparticle size increases, 1/2 pattern charged medium nanoparticles can self-assemble into linear structures, while those with a 1/4 pattern self-assemble into clusters. For very large nanoparticles, both 1/2 pattern and 1/4 pattern charged nanoparticles self-assemble into flaky structures with different connections. By considering the effects of surface charged pattern and nanoparticle size on self-assembly, we found that nanoparticle self-assembly requires a minimum effective charged area. When the local charged area of nanoparticles is less than the threshold, surface charge cannot induce nanoparticle self-assembly; that is, the surface charged pattern of a nanoparticle would determine effectively the self-assembly structure. It is expected that this work will provide guidance for nanoparticle-assisted drug delivery.

Predicting and rationalizing the effect of surface charge distribution and orientation on nano-wire based FET bio-sensors

A single charge screening model of surface charge sensors in liquids (De Vico et al., Nanoscale, 2011, 3, 706-717) is extended to multiple charges to model the effect of the charge distributions of analyte proteins on FET sensor response. With this model we show that counter-intuitive signal changes (e.g. a positive signal change due to a net positive protein binding to a p-type conductor) can occur for certain combinations of charge distributions and Debye lengths. The new method is applied to interpret published experimental data on Streptavidin (Ishikawa et al., ACS Nano, 2009, 3, 3969-3976) and Nucleocapsid protein (Ishikawa et al., ACS Nano, 2009, 3, 1219-1224).

Effect of Surface Charge Distribution on the Adsorption Orientation of Proteins to Lipid Monolayers

The adsorption orientation of the proteins lysozyme and ribonuclease A (RNase A) to a neutral 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and a negatively charged stearic acid lipid film was investigated by means of X-ray reflectivity. Both proteins adsorbed to the negatively charged lipid monolayer, whereas at the neutral monolayer, no adsorption was observed. For acquiring comprehensive information on the proteins' adsorption, X-ray reflectivity data were combined with electron densities obtained from crystallographic data. With this method, it is possible to determine the orientation of adsorbed proteins in solution underneath lipid monolayers. While RNase A specifically coupled with its positively charged active site to the negatively charged lipid monolayer, lysozyme prefers an orientation with its long axis parallel to the Langmuir film. In comparison to the electrostatic maps of the proteins, our results can be explained by the discriminative surface charge distribution of lysozyme and RNase A.

The effect of surface charge distribution on hindered diffusion in pores

An analytic solution is derived for the hindered diffusion of charged, small solutes in charged, cylindrical pores in which the pore wall potential consists of the sum of an average and an oscillatory component. When the oscillatory contribution is absent, the effect of electrostatic interactions on diffusion is negligible. However, when the wall potential or surface charge density varies axially, electrostatic interactions hinder the rate of diffusion significantly, and can stop it completely if “choke points” develop where the solute concentration becomes zero. The degree of hindrance is generally weaker when the electrostatic charge on the pore wall and the charge on the solute have the same signs, leading to a repulsion, than it is in the presence of an attraction. The electrostatic hindrance is also affected by the length scale of the axial variation along the pore wall, becoming stronger as that length grows, until an asymptotic value is reached. The theory for the effect of variations of the electrostatic potential on rates of diffusion is shown to be in good agreement with experimental data taken from the literature. The results here are obtained by using generalized Taylor dispersion theory, and are therefore rigorous predictions of what occurs over times long enough that the solute diffuses through a tube many times longer than a single periodic cell. The electrostatic interactions are calculated using the linear Poisson–Boltzmann equation.

Effect of surface charge distribution on protein transport through semipermeable ultrafiltration membranes

Although several previous studies have demonstrated the importance of electrostatic interactions on the transport of charged proteins through semipermeable ultrafiltration membranes, all of these analyses have implicitly assumed that the protein can be treated as a sphere with uniform surface charge. This is in contrast to the behavior seen in ion exchange chromatography where the presence of localized charge “patches” can significantly affect protein adsorption to the charged resin. The objective of this study was to examine the possible effects of surface charge distribution on the magnitude of the electrostatic interactions that occur during protein ultrafiltration. Data were obtained with cytochrome c and lysozyme, two proteins that have similar size and net charge but different amino acid composition and surface charge distribution. Ultrafiltration experiments were performed with cellulosic membranes, both as received and surface-modified by chemical attachment of a quaternary ammonium functionality to generate a positively charged membrane. Data were obtained over a range pH with the sieving results compared to the measured protein retention on a strong cation-exchange resin. Protein transmission during ultrafiltration through oppositely charged membranes was governed primarily by the net protein charge, with minimal contribution from the surface charge distribution.

Effect of surface charge distribution on the crystal growth of sodium perborate tetrahydrate

Growth and dissolution rates of sodium perborate crystals have been measured in a flow-type single-crystal cell. Sodium perborate grows dendritically at any level of supersaturation and shows growth and dissolution rate dispersion. Both the growth and dissolution rates of sodium perborate were found to be controlled by surface charge distribution which is represented by applied voltages in an electrostatic separator. It was determined that high surface charge dominates the crystal growth rate when compared with low surface charge under identical conditions. The results obtained showed that the formation of dendritic structure is not a function of supersaturation but a function of surface charge. The rate of dissolution of a crystal with a high surface charge is greater than that with a low surface charge.

Effects of Surface Charge Distribution of Proteins in Their Complexation with Polyelectrolytes in an Aqueous Salt-Free System

Complexation between protein and polyelectrolyte was studied by a combination of three main experimental techniques: turbidimetric titration, quasi-elastic light scattering (QELS), and static light scattering (SLS). Proteins of two sorts, lysozyme (Lyz) and ribonuclease (RNase), were chosen by considering the following characteristics: (i) Both proteins have the same number (19) of basic groups; (ii) their distribution is almost homogeneous on Lyz but not on RNase; (iii) there is little difference in the molar mass between both proteins. Potassium poly(vinyl alcohol) sulfate (KPVS) with different molecular weights (MPE) and various degrees of esterification (De) was used as the polyelectrolyte. We employed a salt-free aqueous medium and adjusted it to pH 2, the level of which forces to completion the protonation of all of the basic groups. As the titration of proteins with KPVS proceeded, the absorbance (A) as an indication of turbidity increased linearly and then rapidly at a certain titrant volume, r...